Chemical strategies have been progressively applied to understand and manipulate biological systems. The chemical reactivity of the employed reagents needs to be tuned such that interference with essential biochemical or cellular processes is prevented. Several bioorthogonal reactions have been developed to enable site-selective conjugation of macromolecules with a myriad of probes (e.g., luminescent dyes, photo-responsive moieties etc.),[1] yet the conditional breaking of bonds in the presence of a large heterogeneity of functional groups has received less attention. Cleavable linkers that can be chemoselectively addressed in a biocompatible manner have started to see deployment in disciplines such as biochemistry, proteomics, and cell biology.[2]
One successful application in immunobiology has facilitated the detection of disease-specific T-cell responses within large reservoirs of other cells. T-lymphocytes belong to the cellular arm of the adaptive immune system and are tasked to recognize and eliminate virus-infected or tumor cells. They express a large diversity of clonally distributed surface receptors that govern their specificity toward a cognate antigenic peptide fragment presented by major histocompatibility complexes (MHCs). Recombinantly produced oligomers of the latter heterotrimeric glycoprotein complex can bind to and stain T-cells of corresponding specificity, and the conventional MHC tetramer format has become a cornerstone technology for mapping T-cell responses in basic and clinical research on infectious diseases, autoimmunity, cancer and vaccine development.[3]
Libraries of MHC molecules such as tetramer libraries are among others accessible through synthetic ligands that are released through UV-induced cleavage of the peptide backbone, enabling a novel epitope to refill the evacuated MHC peptide-binding groove.[4] Arrays of the peptide-exchanged MHC tetramers enabled the interrogation of T-cell repertoires, regardless of their functional activity. Technical limitations such as low UV penetration, variability in UV irradiation, and the potential of photo and thermal damage to the protein complexes, highlighted the need for alternative modes of cleavage. Chemoselective peptide exchange, although conceptually feasible, should avoid compromising the replacement epitope with its unprotected functionalities at the amino acid residue side-chains as well as N- and C-termini, or risk the loss of T-cell antigen-recognition.